Chronobiological studies on the hypotensive effect of prostaglandin E2 and arachidonic acid in the rat.

The present study was designed to determine whether biological rhythm variations could be detected in the hypotensive action of prostaglandin E2 (PGE2) and arachidonic acid (AA) in normal rats. Doses of 1.0 microgram kg-1 of PGE2 or 0.5 mg kg-1 of AA were administered to pentobarbital-anesthetized rats at 6 times of the day. Maximal reduction of systolic and diastolic blood pressures was obtained when PGE2 or AA were administered to rats between 0930 and 1200. The lowest falls in blood pressure were found when the same doses of the two substances were injected between 0300 and 0500. Mechanisms to explain these circadian variations are suggested.